The pulmonary circulation system is the only system through which the entire cardiac output passes. The major role of pulmonary circulation is respiratory gas exchange. Therefore, to facilitate this role, pulmonary circulation is a low-pressure, high-flow system. Pulmonary circulation can accommodate any changes in blood flow due to relative passivity and the ability to recruit unperfused vessels. Several factors such as gravity, lung inflation, alveolar surface tension, and blood viscosity can influence pulmonary circulation under both physiological and pathological conditions. For instance, in an upright position, gravity increases both blood flow and ventilation in caudal regions, with a greater change in blood flow than ventilation. As a result, there is a gradual decrease in ventilation-to-perfusion ratios from the apex to the base of the lung in an upright position. The inflation of the lung can compress and distort vessels and alter blood flow through pulmonary circulation. The alveolar surface tension attenuates lung capillary resistance and promotes capillary blood flow. In contrast, an increase in blood viscosity or hematocrit decreases pulmonary flow.
